a uk-based assessment of hybrid pv and solar-thermal ... · pdf filea uk-based assessment of...

Applied Energy 122 (2014) 288–309

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Applied Energy

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A UK-based assessment of hybrid PV and solar-thermal systemsfor domestic heating and power: System performance

http://dx.doi.org/10.1016/j.apenergy.2014.01.0610306-2619 � 2014 The Authors. Published by Elsevier Ltd. Open access under CC BY license.

⇑ Corresponding author. Tel.: +44 20 759 41601.E-mail address: [email protected] (C.N. Markides).

María Herrando, Christos N. Markides ⇑, Klaus HellgardtDepartment of Chemical Engineering, Imperial College London, London SW7 2AZ, UK

h i g h l i g h t s

�We develop a mathematical model to simulate a hybrid solar-thermal system (PVT).� Household electricity and hot water demands covered throughout a year are estimated.� Two key system parameters are varied to optimise the system performance.� Up to 51% of the annual electrical and 36% of the hot water demands are covered.� In addition, 16.0 tCO2 (35% higher than PV-only) are saved over a 20-year lifetime.

a r t i c l e i n f o

Article history:Received 7 May 2013Received in revised form 25 January 2014Accepted 29 January 2014Available online 12 March 2014

Keywords:Hybrid PVDomestic UK energy demandHeat and power provisionSolar energySystem performance

a b s t r a c t

The goal of this paper is to assess the suitability of hybrid PVT systems for the provision of electricity andhot water (space heating is not considered) in the UK domestic sector, with particular focus on a typicalterraced house in London. A model is developed to estimate the performance of such a system. The modelallows various design parameters of the PVT unit to be varied, so that their influence in the overall systemperformance can be studied. Two key parameters, specifically the covering factor of the solar collectorwith PV and the collector flow-rate, are considered. The emissions of the PVT system are compared withthose incurred by a household that utilises a conventional energy provision arrangement. The resultsshow that for the case of the UK (low solar irradiance and low ambient temperatures) a complete cover-age of the solar collector with PV together with a low collector flow-rate are beneficial in allowing thesystem to achieve a high coverage of the total annual energy (heat and power) demand, while maximisingthe CO2 emissions savings. It is found that with a completely covered collector and a flow-rate of 20 L/h,51% of the total electricity demand and 36% of the total hot water demand over a year can be coveredby a hybrid PVT system. The electricity demand coverage value is slightly higher than the PV-only systemequivalent (49%). In addition, our emissions assessment indicates that a PVT system can save up to16.0 tonnes of CO2 over a lifetime of 20 years, which is significantly (36%) higher than the 11.8 tonnesof CO2 saved with a PV-only system. All investigated PVT configurations outperformed the PV-only sys-tem in terms of emissions. Therefore, it is concluded that hybrid PVT systems offer a notably improvedproposition over PV-only systems.

� 2014 The Authors. Published by Elsevier Ltd. Open access under CC BY license.

1. Introduction

A significant increase in energy demand has been observed inthe last decades, driven by an increase in population and/orin the energy use per capita in various regions around the globe.In the UK, energy use per capita has actually decreased by about15% since its peak in the late 1990s, which has more than

compensated for the �10% increase in population over the sameperiod [1]. There is a significant and continuing desire to maintainthis trend, and to diversify and decarbonise the energy supply, thuslowering the reliance on fossil fuels, which arises from the realisa-tion that these are finite resources (giving rise to economic, secu-rity of supply and sustainability concerns) and also that theemissions associated with their use lead to wider environmentaland health problems [1]. Specifically, the UK has made a numberof international commitments and set itself a series of targets forreducing greenhouse gas emissions and increasing the proportionof final energy supplied by renewable sources [2].

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Nomenclature

Abbreviationsa-Si amorphous-crystalline PV modulec-Si mono-crystalline PV modulepc-Si poly-crystalline PV moduleMPP Maximum Power PointNOCT Normal Operating Cell TemperaturePV photovoltaicPVT PV and solar-thermal systemPVT/w PV and solar-thermal water system

SymbolsAc collector aperture area (m2)APV surface area of the PV (m2)cp specific heat capacity of water (=4180 J/kg K)D diameter of the riser tubes (m)DCav percentage of the average overall demand covered by

the PVT system (%)DCE percentage of the electricity demand covered by the PVT

system (%)DCHW percentage of the hot water demand covered by the PVT

system (%)DCwav percentage of the weighted average overall demand

covered by the PVT system (%)eag thickness of the air gap (m)ei thickness of the insulation layer (m)Egrid electrical energy required from the grid over a full year

(kWe h)Eloss electrical energy consumed by the water pump (kWe h)Enet additional (to the PVT-generated) electrical energy re-

quired to cover the short-fall in the household demandover a full year (kWe h)

Eneti additional (to the PVT-generated) electrical energy re-quired to cover the short-fall in the household demandat time step i (We h)

EPV electrical energy produced by the PV-only system over afull year (kWe h)

EPVT electrical energy produced by the PVT system over a fullyear (kWe h)

EPVTi Electrical energy produced by the PVT system at timestep i (We h)

EPVTnet net electrical energy available from the household aftersubtraction of the household’s consumption over a fullyear (kWe h)

ET total annual electricity demand (kWe h)Ewd, Ewe electricity consumption over a day, either during the

week or on the weekend respectively (kWe h)Emaux total CO2 (equivalent) emissions due to the auxiliary

heater over a full year (kg CO2(e))EmcE total CO2 (equivalent) emissions from covering the total

electricity demand from the grid over a full year(kg CO2(e))

EmcHW total CO2 (equivalent) emissions from covering theoverall hot water demand with an equivalent conven-tional system (natural gas boiler, heat pump or electri-cal heater) over a full year (kg CO2(e))

EmPVTE total CO2 (equivalent) emissions incurred to cover thedemand with an installed PVT unit over a full year(kg CO2(e))

EmsE percentage of CO2 (equivalent) emission savings due tothe electricity demand covered by the PVT system (%)

EmsHW percentage of CO2 (equivalent) emission savings due tohot water production by the PVT system (%)

EmsT total CO2 (equivalent) emissions saved over a full yearwhen PVT systems are used to cover the demand(kg CO2(e)/year)

f Darcy–Weisbach (or Moody) friction factor for thewater flow through the pipes

g gravitational acceleration (=9.81 m/s2)Gbi thermal conductance between the absorber, the back/

underside insulation and the environment (W/m2 K)Gca thermal conductance between the PV laminate layer

and the absorber plate (W/m2 K)Gr Grashof numberhair free convective heat transfer coefficient of air at the

back/underside of the PVT module (W/m2 K)hw convective heat transfer coefficient of the water flow in

the pipes (W/m2 K)hwind convective heat transfer coefficient due to wind flow

over the PVT (W/m2 K)J incident global solar irradiance on the tilted PVT collec-

tor surface (W/m2)kair thermal conductivity of air (W/m K)ki thermal conductivity of the insulation layer (W/m K)kw thermal conductivity of water (W/m K)Ks minor loss coefficient for flow through a bendL total length of the water pipe (m)_mc mass flow-rate of water through the collector (kg/s)_ml mass flow-rate of hot water demand (kg/s)_mt mass flow-rate of water through the heat-exchanger lo-

cated in the hot water tank (kg/s)_mtube mass flow-rate of water flowing through the riser tubes

of the collector (kg/s)Mt total mass of water in the hot water tank (kg)NTU Number of Transfer UnitsNu Nusselt numberP PV area covering factor (%)Pnet net electrical power output of the PVT system (W)PPV electrical power output of the PV module (W)PP electrical power consumed by the water pump (W)Pwd, Pwe electrical power demand at a specific time of the day,

either during the week or on the weekend (W)_qag heat flux from the glass cover to the PV laminate layer

due to both conduction and convection through the airgap (W/m2)

_qagab heat flux from the glass cover to the absorber plate dueto convection through the air gap in the uncovered sec-tion without PV (W/m2)

_qaux auxiliary heater power (W)_qbi heat flux through the back/underside insulation

(W/m2)_qbiab heat flux through the back/underside insulation in the

uncovered section without PV (W/m2)_qca heat flux from the PV laminate to the absorber (W/m2)_qcaab heat flux through the absorber plate in the uncovered

section without PV (W/m2)_qct heat addition from the collector to the hot water tank

(W)_qloss heat losses through the hot water tank walls (W)_qload heat removal to cover the domestic hot water demand

(W)_qrPV heat flux from the glass cover to the PV laminate due to

radiation (W/m2)_qrab heat flux from the glass cover to the absorber plate due

to radiation (W/m2)_qsky heat flux loss between the glass cover and the sky due to

radiation (W/m2)_qskya heat flux loss from the glass cover to the sky in the

uncovered section without PV due to radiation (W/m2)_qw heat flux transferred from the absorber to the water

(W/m2)

M. Herrando et al. / Applied Energy 122 (2014) 288–309 289

_qwab heat flux transferred from the absorber plate to thewater in the uncovered section without PV (W/m2)

_qwind heat flux loss due to wind forced convection (W/m2)_qwinda heat flux loss due to wind forced convection in the

uncovered section without PV (W/m2)Q total collector water flow-rate (m3/s)Qaux total auxiliary heating required over a full year (kWth h)Qgas additional amount of heat required (kWth h)QPVT amount of hot water produced by the PVT system over a

full year (kWth h)QT total hot water household demand over a full year

(kWth h)REVA heat resistance of the EVA layer (m2 K/W)Rglue heat resistance of the glue layer (m2 K/W)Rted heat resistance of the Tedlar layer (m2 K/W)Ri heat resistance of the insulation layer (m2 K/W)Rair heat resistance of the air layer (m2 K/W)Re Reynolds numberSt surface area of the hot water tank (m2)T1 free-stream temperature (K)Ta ambient temperature (K)Tas ambient temperature surrounding the hot water tank

(K)Tab temperature of the absorber plate (K)Taba temperature at the front/top surface of the absorber

plate in the uncovered section without PV (K)Tabf temperature at the bottom surface of the absorber plate

in the uncovered section without PV (K)Tbw bulk temperature of the water in the collector (K)Tbwab bulk temperature of the water in the uncovered section

without PV (K)Tcin temperature of the water entering the collector (K)Tcout temperature of the water exiting the collector (K)Tdel delivery temperature of hot water to the household (K)Tg temperature of the glass cover (K)Tga temperature of the glass cover in the section that is not

covered by PV and open to the atmosphere (K)TPVref reference PV cell temperature at an ambient

temperature of Ta = 25 �C and a solar irradiance ofJ = 1000 W/m2 (K)

TPV PV cell temperature (K)TPVmax maximum PV cell temperature reached in a day (K)TPVout temperature of the water entering the uncovered sec-

tion without PV (K)Ts surface temperature (K)

Tsky sky temperature (K)Tsup mains water supply temperature (K)Tt temperature of the water in the hot water tank (K)Ttin temperature of the collector flow at the inlet of the heat

exchanger immersed in the hot water tank (K)Ttout temperature of the collector flow at the outlet of the

heat exchanger immersed in the hot water tank (K)Twin temperature of the water entering the hot water tank

(K)Ut overall heat loss coefficient of the hot water tank

(W/m2 K)(UA)t overall heat transfer coefficient-area product of the

heat exchanger located inside the hot water tank (W/K)VP water flow-rate through the collector (with

1 L/h = 2.78 � 10�7 m3/s)vwind wind speed (m/s)vw bulk water flow speed in the pipes (m/s)

Greekaab absorptivity of the absorber plateaPV absorptivity of the PV moduleb0 temperature coefficient for the PV module (1/K)bair volumetric expansion coefficient of air (ideal gas) (1/K)Dp pressure drop through the closed loop collector (Pa)e effectiveness of the heat exchangereab emissivity of the absorber plateeg emissivity of the glass coverePV emissivity of the PV modulegEref maximum electrical efficiency at reference conditions

when the PV cell is at temperature TPVref (%)gel actual electrical efficiency of the PV system (%)gP pump efficiency (%)lw dynamic viscosity of water (kg/m s)mair kinematic viscosity of air (m3/s)qd diffuse reflectance of the cover plateqw density of water (kg/m3)r Stefan–Boltzmann constant (=5.67 � 10�8 W/m2 K4)sab transmittance of the absorber platesg transmittance of the cover plate (glass)sPV transmittance of the PV module(sa)PV transmittance–absorptance product for the PV module(sa)ab transmittance–absorptance product for the absorber

plate

290 M. Herrando et al. / Applied Energy 122 (2014) 288–309

In residential buildings, where our study is based, solar systemscan provide not only electricity, but also hot water and space heat-ing (and/or cooling) depending on the specific end-user require-ments, and of course the technology used. In particular, solarthermal collectors and photovoltaic (PV) systems are potential on-site renewable energy options; the former can provide heat fordomestic hot water (and also possibly space heating, although thisis not considered in the present study), while the latter can provideelectricity to cover part of the household needs in electrical power.Therefore, there is a significant interest in the development andwider deployment of such systems, since these can form a basisfor the achievement of the aforementioned energy targets, whiledecreasing the fossil-fuel based energy consumption in buildings[3]. However, further performance improvements, as well asinvestment in existing and future solar technologies are necessaryin order to ensure the appropriate infrastructure, and to reduce theassociated costs of these systems thus making them economicallyviable [4,5].

The present paper focuses on the holistic, distributed supply ofboth domestic hot water and electrical power to a representativehousehold through a hybrid solar system. This concept has a verysignificant potential to achieve this goal, which arises from thesynergistic combination within a single arrangement of a PV mod-ule and a solar thermal collector sub-system. This combination al-lows electricity and hot water to be generated simultaneously,while reducing the efficiency deterioration experienced by the PVmodule at elevated temperatures by cooling the PV module witha flow of water that is then used for the provision of hot water.The quantities of hot water and electricity used in householdsare strongly dependent on user behaviour. Therefore, in order toproperly size and design a representative hybrid solar system fordomestic heating and power, it is very important to define and tocharacterise these profiles [6].

The overall aim of this paper is to assess the annualised perfor-mance of a hybrid PV and solar thermal (PVT) system installed onthe roof of an average three-bedroom terraced house located in


London, UK. The interest here is in investigating of the role ofimportant system parameters in maximising the supply potentialof both electricity and hot water in this particular scenario. Anadditional motivating factor is to maintain simplicity of design,leading to a minimisation of systems costs to the end-user. For thisreason a commercially available PVT unit is chosen, and the unit’soperational performance within an overall system for the provisionof hot water and electricity generation are simulated over thecourse of a full year. This design is compared to a reference casebased on the same house using conventional technologies, consist-ing of a combination of natural gas boilers, heat pumps and electri-cal heaters for hot water, and electricity bought from the grid.

In Section 2 we proceed to discuss the concept of a hybrid PVTsystem, review the literature and previous work in this area, andpresent a number of currently available commercial systems, alongwith a summary of their performance and cost. In Sections 3 and 4we present the modelling methodology employed in our study, anddiscuss how the various system parameters where obtained orestimated. Also presented are details of the parametric analysisundertaken to evaluate the performance of our chosen PVT systemfor different design parameters; the aim of this analysis being tofind the parameters which optimise the PVT system for our sce-nario of interest, i.e. the supply of domestic electricity and hotwater to an average household in London. Following this, Section 5contains the main results from the present study, together with arelevant discussion and, finally, the main conclusions from thiswork are stated in Section 6.

Fig. 1. Summary of commercially available hybrid PVT systems: PVT/a (bluediamonds), PVT/w (red squares) and concentrated PVT (green triangles), all in termsof their thermal vs. electrical output (W/m2). (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

2. Hybrid PVT systems

2.1. Combined solar heat and power

Most of the solar radiation absorbed by a PV cell is converted toheat, increasing the temperature of the cell and decreasing its elec-trical efficiency [7–10]. To overcome this problem, solar cells canbe cooled by a flow of a suitable fluid (gas or liquid), decreasingtheir temperature and improving their efficiency, while producinga useful thermal output. The synergistic combination of the im-proved electrical output and the associated heat-provision poten-tial (for hot water provision and/or space heating) havemotivated the development of hybrid PV/thermal (PVT) concepts[7–10], which have emerged as holistic solar energy solutions thatcombine a PV module for electricity generation, coupled with aheat exchanger arrangement and a coolant circuit containing aheat transfer fluid for heat provision from the same collector area[8,11–14].

The total energy output (electrical plus heat) of a hybrid PVTsystem depends on several factors, such as: the configuration de-sign and heat extraction arrangement employed; the solar irradi-ance, ambient temperature and wind speed; and the operatingtemperatures of a number of important components. In mostapplications the electrical output is typically the main priority, inwhich case the operating condition of the heat transfer arrange-ment is adjusted to optimise electrical performance. Specifically,the cooling fluid (typically air or water) in the heat transfer circuitis kept at a low temperature in order to avoid an otherwise unde-sirable decrease in the electrical efficiency of the PV cell [8,10–12].This constraint for the heat transfer fluid to exit the collector at lowtemperatures in order to allow higher electrical outputs imposes alimit on its posterior use for heating purposes. On the contrary, ifthe system were designed to provide higher fluid temperaturesat the PVT unit outlet for use in applications such as water heating,then the electrical efficiency would decrease [8,10,12,15]. Hence, adesign conflict arises between the electrical and thermal perfor-mance of hybrid PVT systems, and a trade-off is needed depending

on the end-user needs and the local solar and environmental con-ditions. This conflict along with the elevated costs of PVT, are thetwo main reasons why these systems are currently not as widelyemployed as separate, individual PV and solar thermal collectorequivalents [10].

2.2. Commercial systems and system selection

Although hybrid systems are not yet a fully mature technologyand their commercialisation is still in its early stages, a small num-ber of manufacturers are nowadays producing PVT air (PVT/a) andPVT water (PVT/w) systems, as well as concentrating systems. Theperformance of a variety of commercially available systems, interms of both thermal and electrical output, along with the corre-sponding manufacturers is shown in Fig. 1.

This paper focuses on PVT/w systems, which feature a PV mod-ule placed in thermal contact with a water-based solar thermal col-lector. PVT/w systems are considered the most efficient way ofpreheating water over the course of a whole year [7,8,10]. Mostof the PVT/w systems in Fig. 1 (red squares) are based by thermallyattaching mono-crystalline PV modules on top of flat plate solarcollectors [16–19]. It is important to note that most manufacturersof PVT/w systems have developed their systems by modifyingcommercially available solar collectors to include the PV moduleon its absorber surface [14]. The utilisation of the surface area thatis exposed to the solar radiation is of primary importance andinterest. When the PV cell is placed above the thermal collectorsection, in order to achieve high electrical performance, the pres-ence of the PV leads to a reduction of the heat flux into the collec-tor fluid circuit, reducing the module’s thermal efficiency. Previousstudies (e.g. [20,21]) investigated the trade-off between larger cov-erage areas for increased electricity generation and smaller cover-age areas when the priority is hot water production, yet this trade-off is sensitive to the solar and environmental conditions at thegeographical location of installation.

Of the different types of PVT/w system, configurations incorpo-rating sheet-and-tube collectors are considered particularly prom-ising options for domestic hot water production [22]. Even thoughthey perform marginally worse than channel collectors (typicallyby �2%; [22]), they are a good option in terms of thermal efficiency(58%; [22]) and, importantly, are a simple and low-cost configura-tion to manufacture, as they only require the integration of a stan-dard PV panel with a standard thermal collector with no majormodifications, relying on mature and widely available technolo-gies. In addition, the results in Ref. [22] indicate that although asheet-and-tube collector with two covers has a slightly higher


thermal efficiency at elevated water temperatures compared to asingle-cover collector, its electrical efficiency deteriorates consid-erably due to the second cover. Thus, we consider the single-coversheet-and-tube PTV/w collector as a highly suitable starting pointtowards the achievement of a favourable trade-off between ther-mal and electrical output, at a reasonable cost.

Numerous studies [10,12,13,16,17,23] have compared the elec-trical and thermal performance of PVT systems to that of separatePV modules and solar thermal collectors. An important finding is,once again, that beyond the choice of the configuration, theperformance of these systems is highly dependent and geographi-cal location [23]. Tripanagnostopoulos et al. [8] found that PVT/wsystems have slightly higher electrical efficiencies compared tostandard PV modules, however, other studies concluded that theelectrical output of PVT systems is actually lower (e.g. by �38%in Ref. [11]) because of the higher operating temperatures andthe additional glazing. Further, PVT collectors can be less efficientthan conventional solar thermal collectors in extracting heat, dueto the reduced conductivity of the absorber and limitations im-posed by the presence of the PV module. The studies of Tripanag-nostopoulos [10] and Vokas et al. [23] are in agreement withrespect to this point, that latter showing a lower value (of around9%) for the thermal efficiency of a PVT system compared to conven-tional solar thermal collectors. Another study undertaken in Tai-wan (see Ref. [15]) reported that the thermal and electricalefficiencies of a hybrid PVT/w system was better than the efficien-cies of separate, conventional solar water-heating collectors andPV modules, with a thermal efficiency of up to 38%, a primary-en-ergy saving efficiency of more than 60% and a PV efficiency of 9%.

In any case, a distinctive characteristic of PVT collectors is thedual output of electricity and hot water, so that even in the casethat both the yields are slightly lower in a PVT system than in con-ventional PV and solar thermal systems (as in, e.g. Ref. [14]), thePVT system generates both heat and electricity from the same sur-face area, as opposed to separate, individual side-by-side PV panelsand solar thermal collectors [22,24]. Hence, an additional advan-tage appears when the available external building surface is lim-ited, above and beyond the fact that PVT systems constitute anintegral unit, which is considered more aesthetically pleasing, pro-viding greater architectural uniformity than separate systems witha different appearance [10,13,22].

Another aspect of hybrid technologies that is of particular inter-est concerns the circulation of the cooling fluid. Thermosyphonsystems have been identified as a good option in warm locationswith subtropical or temperate climate conditions [9,25]. However,at higher latitudes (i.e. colder regions such as the UK, which is ofdirect interest to the present study) the outdoor temperaturescan remain below the freezing point of water, typically formore than a third of the year, in which case the addition of anti-freeze liquid into the fluid circuit reduces their thermal perfor-mance by about 15% [11,26], and makes them a less appropriatesolution.

In conclusion, the extent and specific area layout/coverage ofhybrid PVT systems, as well as the mode of the fluid circulation, ap-pear as two specific system parameters of particular interest, as isthe specific evaluation of the performance of hybrid PVT system ina particular climate, which in this work is the UK. Fig. 1 indicatesthat amongst the commercially available PVT systems, an excellentoption in terms of combined thermal and electrical output is theconcentrating PVT system of Zenith Solar but this unit is not suit-able for roof-top domestic installation as it requires solar tracking.Therefore, two very appropriate options amongst the availablePVT/w systems are the systems manufactured by PVTWins [18]and by ENERGIES-SOL [16]. In the present paper we have selectedthe latter for further analysis and examination, as it provides the

highest electrical output while maintain a similar thermal outputper unit surface area.

3. Modelling methodology

The core components of the complete hybrid PVT/w system thatwe will focus on in this paper are: (i) the PVT collector; (ii) a hotwater (thermal) storage tank; (iii) an auxiliary heater necessaryto meet the temperature requirements of the end-user when thesolar supply is insufficient; (iv) a water circulator pump; and (v)interconnecting pipework [7,14]. In our approach, we have splitthe overall system into two separate sub-systems: (i) the PVT unitwith its active, closed water loop, including the pump (Section 3.1);and (ii) the hot water storage tank and the auxiliary heater (Sec-tion 3.2). The two sub-systems interface at the storage tank, wherethey are connected by a heat exchanger located inside the tank. Inthis heat exchanger heat is transferred from the water circuit flow-ing through the solar collector to the water in the tank.

3.1. PVT/w unit modelling

3.1.1. PVT/w unit descriptionThe two main components of the PVT/w unit are the PV module

and the rest of the solar collector, which can be further divided intothe glazing, the thermal absorber, the riser water tubes and theinsulation layer. Similarly to previous PVT modelling attempts[3,22,27,28], energy balance equations were written in order tocalculate the heat transfer rates and temperatures throughoutthe unit. The equations were applied separately to each layer ofthe PVT unit, instead of using global equations to find the averageabsorber plate temperature and the energy flows [24,29]. This al-lowed an estimation of the average temperatures of all the sepa-rate unit layers with the aim of defining the system state moreaccurately. Based on the large Fourier numbers (characterisingthe ratio of thermal conduction to unsteadiness) of our problem,a quasi-steady assumption was made, according to which the pa-nel was assumed to be in thermal steady-state at each time instant.

In more detail, the PVT unit modelled in this paper is based onthe commercially available hybrid system ENERGIES-SOL, as pre-sented in Ref. [16]. In addition, a few typical parameters wereneeded from the literature [3,22,24,30,31], in order to fully definethe system. The ENERGIES-SOL unit consists mainly of (from top tobottom): a transparent cover (glass), an air gap, a mono-crystalline(c-Si) PV module, an EVA encapsulating film, an absorber–exchan-ger which transforms the solar radiation to heat and transfers it tothe collector fluid, and a layer of insulation material at the bottom(Fig. 2). The backside/underside, and also side insulation (notshown here) layers reduce heat losses, while improving structuralstrength. The absorber–exchanger consists of a sheet-and-tubeheat exchanger in which water flows in parallel pipes (eleven cop-per riser tubes) from the header inlet pipe to an outlet pipe on theupper side of the collector that collects the warm fluid [32]. Thetransparent cover is a single glass sheet with a thickness of3.2 mm [16,22,24].

Although the commercially available ENERGIES-SOL PVT collec-tor unit is completely covered by PV modules over its entire sur-face area, our model of this unit splits the collector into twosections (Fig. 3): (i) a PV-covered section that occupies a (variable)fraction P of the total collector area; and (ii) an uncovered sectionthat occupies the remainder of the area. This was done in order tostudy the effect of coverage of the unit with PV, by allowing a di-rect adjustment of the relative importance of the unit’s electricaland thermal outputs.

As shown in Fig. 3, the overall PVT arrangement comprises an ac-tive closed-loop system in which, in normal operation, the collector

Fig. 2. Referring to the PV-covered section of the PVT collector (#2 in Fig. 3): (a) PVT collector cross-section. (b) PVT layers: 1. Tempered glass (high transmittance), 2. EVAencapsulating film, 3. c-Si PV cells, 4. EVA encapsulating film, 5. Adhesive plus back-sheet Tedlar, 6. Aluminium absorber plate plus solar collector, 7. Insulating layer. Takenfrom Ref. [25]. The uncovered collector section is identical, but without the PV, EVA, adhesive and Tedlar layers (3–5).

Fig. 3. Schematic diagram of the full PVT domestic hot water system: 1. Solar collector/PVT unit, 2. Section (area fraction P = APV/Ac) of the collector covered by PV, 3. Coolingwater flow tubes, 4. Water pump (flow-rate _mc , VP), 5. By-pass, 6. Tank heat exchanger, 7. Water storage tank, 8. Auxiliary heater, 9. Mixing device for final temperaturedelivery regulation.


outlet flow (flow-rate _mc and temperature Tcout) enters the heat ex-changer located inside the hot water storage tank (flow-rate_mt ¼ _mc and temperature Ttin = Tcout), heats the water in the tank

(mass Mt and temperature Tt), exits the tank (temperature Ttout)and returns to the inlet of the solar collector (temperatureTcin = Ttout) to be heated again. A bypass valve is required to controlthe temperature of the water leaving the collector and entering thetank (Ttin = Tcout), and to ensure that this stream only heats (anddoes not cool) the water in the hot water storage tank. This bypassis controlled by a differential on/off solenoid valve that sends thecollector fluid flow to the tank only when the outlet flow tempera-ture from the collector is greater than the temperature of the waterin the storage tank, or Tcout > Tt. If this is not the case, the fluid isreturned to the collector via the bypass connection for furtherheating, such that Tcin = Tcout [32].

The PVT/w model developed in the present work has beendeveloped under the following assumptions:

� The fraction of solar irradiance that is not converted intoelectricity in the PV cells and is not lost to the environment istransmitted to and absorbed by the absorber plate in the formof heat [31].� The PV cells and the absorber plate are in perfect thermal con-

tact [31].� Heat transfer (losses) at the sides of the PVT collector are negli-

gible [28].� The heat capacities of the PVT system components (PV cells,

Tedlar and insulation) are negligible compared to the heatcapacity of water [30].� The optical properties of the glass cover, absorber plate and PV

cells are constant; for simplicity, the transmittance of the EVAlayer is assumed to be unity [30].� The water flows through the PVT collector tubes are uniform,

with the total mass flow-rate divided equally among all (eleven)tubes running through the collector [3].

� The pipes connecting the PVT unit with the water storage tankare well insulated, such that there are no heat losses to the envi-ronment [31].� The system is in steady state [22,30], based on the justification

made early in this section.

3.1.2. PVT unit model equationsIn each section (PV-covered and uncovered) and for each layer

in that section (see Figs. 2 and 3), an energy balance consideringradiative, convective and conductive thermal exchanges betweenthe layers, the cooling water flow and the environment (where rel-evant) is applied. A spatially averaged temperature is taken acrossthe different sections of each layer [30]. The equations that resultfrom these balances, and which are detailed in the proceeding sec-tions below, are solved at 48 half-hourly time steps (i = 1–48) in or-der to estimate the performance of the system throughout a day.

3.1.2.1. PV-covered section. After neglecting the absorption of solarradiation by the front/top layer (glass cover), given the very lowabsorptivity of glass (ag = 0.05), the energy balance for the glasscover is (using heat fluxes, i.e. per unit area),

_qskyAPV þ _qwindAPV ¼ _qrPV APV þ _qagAPV : ð1Þ

The first heat flux term on the LHS of Eq. (1) is given by [33,34],

_qsky ¼ egr T4g � T4

sky

� �; ð2Þ

with Tsky ¼ 0:0552T1:5a [28,33], while the second term is given by,

_qwind ¼ hwindðTg � TaÞ: ð3Þ

Various expressions are given in different sources for the esti-mation of hwind [3,28,33,35]. These expressions do not differ signif-icantly, and so it was decided to use the expression that providesintermediate values, within the range of the various predictions,which is hwind = 4.5 + 2.9vwind, and vwind is taken here as 5 m/s


[36]. Since the wind speed is not constant, an average between theforced convection coefficient from Eq. (3a) and a free convectioncoefficient (without wind) is considered.

A radiative heat balance between two infinite parallel plates canbe applied to the first RHS term in Eq. (1), which describes the heattransfer from the glass cover to the PV laminate (adapted from Ref.[33]), such that,

_qrPV ¼rðT4

PV � T4gÞ

1=ePV þ 1=eg � 1: ð4Þ

Finally, heat can be transferred from the glass cover through theair gap to the PV laminate by convection, conduction, or both. Theconvective heat transfer coefficient can be estimated from knowl-edge of the Nusselt number (Nu = hwD/kw), which can be related bymeans of correlations to the Grashof number ðGr ¼ gbairDTL3=v2

airÞ[33]. A 5 mm plate spacing (gap size) leads to a value for Gr (�68)that is 4 orders of magnitude lower than that required in commoncorrelations for Nu (i.e. 104–107). Thus, natural convection withinthe air gap is neglected and, consequently, it is assumed that heattransfer through the air gap is mainly due to conduction,

_qag ¼kair

eagðTPV � TgÞ: ð5Þ

Having considered the glass cover (first layer), the energy bal-ance for the PV laminate below is,

ðsaÞPV JAPV ð1� gelÞ ¼ APV ð _qrPV þ _qag þ _qcaÞ: ð6Þ

Following Ref. [33] the transmittance–absorptance product iscalculated from ðsaÞPV ¼

sgaPV1�ð1�aPV Þqd

with qd = 0.16, while J is the inci-dent global solar irradiance on the tilted PVT collector surface, whichhas been placed at the optimal inclination angle and orientationthat maximises the annual solar irradiation in London, UK (seeSection 4.2). The PV efficiency depends on the module temperatureand is estimated from the expression gel = gEref[1 � b0(TPV � TPVref)][37], where TPVref = 25 �C at an irradiance of 1000 W/m2 (manufac-turer-stated values) and b0 is also given in the technical specifica-tions of the ENERGIES-SOL system being considered here.

Furthermore, the heat flux from the PV module to the absorberplate on the RHS of Eq. (6) can be found from,

_qca ¼ GcaðTPV � TabÞ: ð7Þ

The PV laminate is composed of the following layers: a c-Si wa-fer, which has a very high thermal conductivity compared with theother layers (with k � 149 W/m K) and is therefore neglected; a50 lm thick layer of highly conductive glue that minimisesthermal resistance (k = 0.85 W/m K); a 0.1 mm thick PE-Al-Tedlarlayer (k = 0.2 W/m K); and a 0.5 mm EVA layer (k = 0.35 W/m K)[16]. Based on these thicknesses and thermal conductivities, theheat conductance (inverse of resistance) of the entire PV laminatelayer as this appears in Eq. (7) is determined as

Gca ¼ ðREVA þ Rted þ RglueÞ�1 ¼ ð5�10�4

0:35 þ 1�10�4

0:20 þ 5�10�5

0:85 Þ�1¼ 500 W=m2 K.

In addition, the heat flux term _qca in Eqs. (6) and (7) is related tothe heat transferred from the absorber layer to the water flow plusthat lost through the backside/underside insulation layer to theenvironment, or,

_qca ¼ _qbi þ _qw; ð8Þ

where the heat flux lost to the environment through the backsidelayer insulation is,

_qbi ¼ GbiðTab � TaÞ: ð9Þ

The heat transfer conductance between the absorber and the envi-ronment (through the backside layer insulation) that appears in Eq.

(9) is calculated from Gbi ¼ ðRi þ RairÞ�1 ¼ ðeikiþ 1

hairiÞ�1, where hair is

the free convective heat transfer coefficient of air at the undersideof the PVT module assuming that the effect of the wind is negligible,such that this can be calculated from Eq. (3) with vwind = 0 m/s.

Similarly, the convective heat transfer from the absorber layerto the cooling water flow can be related to the temperature differ-ence between the absorber plate and the (bulk) water stream,

_qw ¼ hwðTab � TbwÞ: ð10Þ

Here, the bulk water stream temperature Tbw is estimated as anaverage temperature between the water entering the collector Tcin

and the water exiting the PV covered part TPVout (refer to Fig. 3),i.e. Tbw = (TPVout + Tcin)/2, and the heat transfer (flux) from the absor-ber to the water can be estimated from,

_qw ¼ _mccpðTPVout � TcinÞ: ð11Þ

In order to evaluate the convective heat transfer coefficient of thewater flow in the pipes hw that appears in Eq. (10), the nature of theflow condition must be established. This is done by considering theReynolds number (Re ¼ qtD=lw ¼ 4 _mtube=pDlw). In the presentinvestigation we have confirmed that the condition Re < 3000 is al-ways met, such that the flow is always laminar. For laminar flow,and assuming fully developed conditions, the appropriate heat trans-fer coefficient correlation is used, which is Nu = hwD/kw = 4.36 [34].

Finally, the PVT unit electrical power output is calculated fromthe irradiance normal to the collector plane J,

PPV ¼ JAPVgel; ð12Þ

and the net power output Pnet accounts also for the power con-sumed by the water pump PP and the electrical demand at that timeof the day, Pwd or Pwe, depending on whether it is a weekday or aweekend day, such that,

Pnet ¼ PPV � PP � fPwd; Pweg: ð13Þ

3.1.2.2. Uncovered collector-only section. The energy balance for thefront/top layer (glass cover) of the solar collector section that is notcovered by PV is very similar to the PV-covered equivalent statedabove, but in this case applied to the area Ac (1 � P),

Acð1� PÞð _qskya þ _qwindaÞ ¼ Acð1� PÞð _qrab þ _qagabÞ; ð14Þ

where P = APV/Ac, and the heat flux terms _qskya, _qwinda, _qrab and _qagab

are calculated using the same method as for the covered section,i.e. Eqs. (2)–(5), with the absorber layer replacing the PV layer.

Proceeding to the next layer in this section, the energy balancefor the absorber plate is,

ðsaÞabJAcð1� PÞ ¼ Acð1� PÞð _qrab þ _qagab þ _qcaabÞ; ð15Þ

where (sa)ab is calculated from the expression provided below Eq.(6) with the absorber replacing the PV, i.e. ðsaÞab ¼

sgaab1�ð1�aabÞqd

, and_qcaab is calculated similarly to Eq. (9) with Gca assumed equal to thatin the covered section since the only difference is the thermal con-ductivity of the c-Si layer that is considered negligible.

Similarly, this heat _qcaab can be transferred to the water _qwab orlost through the insulation layer _qbiab,

_qcaab ¼ _qbiab þ _qwab; ð16Þ

where _qwab and _qbiab are again calculated using the same methods asin the covered section, Eqs. (9)–(11).

3.1.2.3. Power and pressure drop. The household electrical demand(see Section 4.1 for details) is imposed as an input to the modeland the power consumed by the water pump of the PVT systemis calculated from,

PP ¼DpQgp

; ð17Þ


where gP is taken as constant and equal to 0.85 [38], and Dp can beevaluated as the superposition of major losses caused by frictionand minor losses in the bends and fittings due to the flow of water,

Dp ¼ fLD

12qwv2

w

� �þX

KS12qwv2

w

� �: ð18Þ

In this work the flow is laminar in all conditions (justificationgiven below Eq. (11)), such that f = 64/Re.

3.2. Water storage tank modelling

On the other side of the system from the PVT unit (refer toFig. 3), cold water from the mains (temperature Tsup) flows intothe tank (temperature Twin = Tsup) where it is mixed with the wateralready contained therein to a temperature Tt. When there is a de-mand from the household for hot water, water (temperature Tt) isdrawn from the tank. An auxiliary heater is placed at the outlet ofthe hot water tank, which is necessary in order to meet the tem-perature requirements of the end-user when the solar supply isinsufficient. Finally, in the case that hot water is required at a tem-perature lower than the tank water temperature (Tdel < Tt), a mixeris introduced to cool the water from the tank before it is directed tothe house. The mixer is not considered in our model since, in theinterests of studying the most demanding scenario and for simplic-ity of design, a constant hot water supply temperature catering to aconstant demand temperature of Tl = Tdel = 60 �C is used.

In addition to the assumptions listed previously, the followingassumptions are applied to the tank model:

� The tank only loses heat [31], e.g. it does not gain solar heat.� The storage tank is fully mixed, that is, there is no stratification

due to the forced mode of operation and continuous supply ofhot water load [3,30,39].

An energy balance on the tank can be discretised in time andsolved at 30-min intervals, such that the water temperature inthe tank at each time-step Tt (i + 1) is calculated from the variousheat fluxes at the previous time-step [39], that is,

MtcpTtðiþ 1Þ � TtðiÞ

Dt¼ _qctðiÞ � _qlossðiÞ � _qloadðiÞ: ð19Þ

In Eq. (19), the heat losses through the walls at time-step i canbe evaluated by,

_qlossðiÞ ¼ StUt ½TtðiÞ � Tas�; ð20Þ

and the energy removal to supply the domestic hot water demandis,

_qloadðiÞ ¼ _mlðiÞcp½TtðiÞ � Twin�; ð21Þ

where water enters the tank is at the mains supply temperatureTwin = Tsup = 10 ± 2.6 �C in the UK [40].

To evaluate the rate of thermal energy transfer from the collec-tor to the water storage tank at time (i), the bypass setting (#5 inFig. 3) must be accounted for, since heat is only added to the tankwhen the temperature of the water exiting the collector is higherthan the temperature of water in the tank, Tcout(i) > Tt(i). Otherwise,there is no heat addition, that is, _qctðiÞ ¼ 0. The heat added to thetank _qctðiÞ is evaluated by considering the heat transfer acrossthe heat exchanger located inside the tank. According to Ref.[35], the most practical approach towards heat exchanger designfor solar systems is based on the NTU-effectiveness (e) method.Thus, the rate of energy added from the collector to the tank,_qctðiÞ, is calculated from,

_qctðiÞ ¼ e _mccp½TcoutðiÞ � TtðiÞ�; ð22Þ

and the temperature at the outlet of the heat exchanger immersedin the tank, Ttout, is,

TtoutðiÞ ¼ TcoutðiÞ � e½TcoutðiÞ � TtðiÞ�: ð23Þ

As this system is a closed loop, this will also be the temperatureentering the collector at next time-step,

Tcinðiþ 1Þ ¼ TtoutðiÞ: ð24Þ

The effectiveness e can be predicted directly from the character-istics of the heat exchanger and the relevant flows. The equationsto determine this parameter can be simplified in situations wherethe temperature on one side of the heat exchanger are constant, aswith a coil in a (non-stratified, uniform) tank, as is assumed to bethe case in the present study. Then, the effectiveness e and NTU arecalculated from:

e ¼ 1� e�NTU ; NTU ¼ ðUAÞtð _mcpÞs

¼ ln1

1� e: ð25Þ

The overall heat transfer coefficient in the tank heat exchanger,ignoring conduction through the walls, is,

1ðUAÞt

¼ 1hiAiþ 1

heAe; ð26Þ

where the subscripts i and e refer to the fluid flow inside and thefluid outside the coil, respectively. In the particular case of a coil im-mersed in a water tank, it is expected that hi he due to the (en-hanced) forced convection experienced as a result of the higherflow speeds inside the coil, while the water in the tank experiencesonly free convection. Therefore, the limiting resistance is that due tofree convection in the tank. The correlation used to calculate thiscoefficient for immersed (external flow) geometries is [34],

Nu ¼ hLk¼ CRan; ð27Þ

where C = 0.52, the Rayleigh number (Ra = Gr�Pr = gb(Ts � T1)L3/ma)is based on a characteristic length L of the geometry (in this case thetube diameter D), and the exponent n = 1/4 for laminar flow.

Finally, as stated previously, an auxiliary heater is used in orderto cover the hot water demand requirements when the storagetank temperature Tt (here in �C) is lower than 60 �C. The additionalheat necessary is,

_qauxðiÞ ¼ _mlðiÞcp½60� TtðiÞ�: ð28Þ

3.3. Annual performance calculations

The model developed to assess the PVT system performanceprovides results on a diurnal basis. These results are then used tocalculate the monthly outputs of the system for the differentmonths of the year, with the final aim of obtaining total annualoutputs. The input data to the model are average daily profilesof: solar irradiance, ambient temperature, hot water demand andelectricity demand, all over a 24-h period. These inputs varydepending on the month of the year, and the electricity demandalso depends on whether it is a weekday or weekend day. Hence,the PVT system model is run 24 times (12 months; one averageweekday and one average weekend day per month), and the out-puts are compiled and suitably weighted for weekdays and week-ends to obtain total monthly and annual performance results [41].Each daily run comprises 30-min intervals, such that the 24-h per-iod is divided into a set of 48 time steps, assuming that all param-eters and variables are constant during each time interval. Then,for example, for the total yearly electricity demand,


ET ¼X12

1

26112

X48

i¼1

EwdðiÞ þ10412

X48

i¼1

EweðiÞ( )

; ð29Þ

where Ewd(i) and Ewe(i) are the average household electricity de-mands on a weekday and a weekend day respectively, and i repre-sents each half-hour interval for which the model was run. Thisequation is applied similarly to each output or other variable ofinterest from the PVT system model to obtain total annual results,which are then used for the comparison of the performance of dif-ferent system configurations.

Two important PVT system performance indicators are the per-centages of the household electricity and hot water demands cov-ered by the PVT system outputs. The total annual values of thesekey performance indicators are calculated from the results of equa-tions such as Eq. (29), as follows:

DCEð%Þ ¼EPVT � Eloss

ET� 100; DCHWð%Þ ¼

Q PVT

Q T� 100; ð30Þ

where EPVT and QPVT are the raw electrical and net thermal energy(for hot water production) outputs of the PVT system, and Eloss arethe electrical pumping losses, all over a full year. In addition,ET and QT are the total annual electricity and hot water householddemands, respectively. Note that EPVT is related to the (typically�ve) net electrical energy available from the PVT-supportedhousehold EPVTnet, via EPVTnet = EPVT � Eloss � ET.

In order to compare the integrated (electricity plus heat) perfor-mance of the different configurations studied, two additionalparameters are considered. These are the average percentage ofdemand covered (DCav) and the weighted average percentage ofdemand covered by the PVT system throughout the year (DCwav):

DCavð%Þ ¼DCE þ DCHW

2; DCwavð%Þ ¼

ET DCE þ QT DCHW

ET þ Q T; ð31Þ

which are average measures of the combined electrical and thermalhousehold demands covered by the PVT. The former is a directarithmetic mean of the two covered demands, while the latter is aweighted mean that takes into account the contribution of the ac-tual amounts of electrical and thermal energy demands.

3.4. Environmental assessment

Beyond studying the performance of the modelled PVT unit, afurther goal of this work is to estimate the possible emissions sav-ings made by the installation of such a system compared to theemissions associated with the use of conventional means, basedon the common current practices of buying the electricity fromthe grid, and using a boiler, heat pump or electrical heater to satisfythe hot water demand (see Section 4.3).

With regards to electricity, the emission saving is due to the dif-ference between the emissions associated with the purchase of allelectricity from the grid (EmcE) and the emissions incurred after aPVT unit is installed (EmPVTE), while the hot water saving arisesfrom the reduction in the required primary fuel for heating, fromthe conventional levels (EmcHM) to the lower auxiliary heatinglevels needed by the PVT system (Emaux),

EmsEð%Þ ¼EmcE � EmPVTE

EmcE� 100;

EmsHWð%Þ ¼EmcHW � Emaux

EmcHW� 100: ð32Þ

The hot water emission terms are calculated, by considering atypical/average UK household heating mode that features a mixof gas boilers, heat pumps, etc., representative of the UK. This isdiscussed in Section 4.3.

4. Model parameters

4.1. Reference house

A reference house is required for the estimation of the hot waterand electricity consumptions, as well as the available roof for theinstallation of the system. According to Ref. [42], the most commontype of house in London is a terraced house, with an average num-ber of about 3 bedrooms. An average number of 4 inhabitants is as-sumed; 2 adults and 2 children. A floor area of around 70–90 m2 isexpected in this house [43]. The average available roof area for theinstallation of solar systems is 15 m2 [44].

4.1.1. Electricity demand profileThe electricity demand profile of an individual dwelling is more

difficult to define as it depends strongly on the activities of theoccupants as well as the electrical appliances available and theirassociated use [45]. A number of different models and profiles havebeen reviewed, with studies undertaken in Sweden [6,41] and alsoin the UK [45]. The UK study, which includes a model developed bythe Centre for Renewable Energy Systems Technology (CREST), hasbeen selected here for our calculations because it combines boththe patterns of active occupancy (considering when inhabitantsare at home and awake) and their daily activity profiles, coveringall commonly used appliances in a domestic dwelling in the UK.Furthermore, the configuration of these appliances considers theirmean total annual energy demand along with their associatedpower-use characteristics, such as their steady-state consumptionor typical use cycles. The active occupancy reflects the naturalbehaviour of real people in their daily lives and it is representedas an integer that varies throughout the day in a pseudo-randomway. In order to create the active occupancy profiles, data derivedfrom the UK 2000 Time Use Survey (TUS) about how people spendtheir time in the UK is used. Finally, the share of appliances as wellas their correlated use is considered. The temporal resolution is 1-min with a 365 day simulation, which yields 525600 data points[45].

This model [45] was validated against the electricity demandof 22 dwellings in the town of Loughborough in the EastMidlands (UK), recorded over a 1-year period. It also takes intoaccount lighting, by considering the level of natural daylightwhen calculating the electrical light needed [45]. As it is availableonline as an Excel file [46], it is possible to estimate the electricitydemand profile for the reference house studied in the presentwork by introducing 3 inputs: the number of residents in thehouse, the month, and whether it is a weekday or a weekend.The appliances are then randomly allocated and the electricitydemand is estimated. An example of the output provided by thesimulation for a July weekday in a house with 4 occupants isshown in Fig. 4(a).

The average annual electricity consumption per dwelling overthe whole set of synthetic data is 4124 kW h, which is close tothe 4172 kW h mean demand from the measured data (for the 22measured buildings) [45]. In the simulation carried out in the pres-ent work the annual consumption is taken as 4500 kW h, a value inaccordance to the previous data obtained and also in the range of4400–4500 kW h/year given in Ref. [41].

The profile shown in Fig. 4(a) is in agreement with a studyundertaken in England of the domestic load of 8 homes, in whichthe typical demand had a base load of �0.5 kW, with peaks inthe morning and evening of up to 4 kW [47]. Although the 1-minresolution is more accurate in reflecting the actual consumption,data averaging was done to find mean load values in 30-min inter-vals in the interest of data management.

(a) (b)

Fig. 4. (a) Electrical power load profile (1-min resolution) for a UK house of 4 inhabitants over a weekday in July [55]. (b) Diurnal hot water consumption profile used in thiswork, according to Ref. [53].


4.1.2. Hot water demand profileThe hot water demand varies considerably depending on the

consumer and throughout the day/month/year, with a higher con-sumption but at a lower temperature requirement in the summer,and strong day-to-day variations. Nevertheless, it is possible forsimplicity of analysis and greater transferability of the results todefine and employ a repetitive average daily load profile and a dai-ly averaged value for further investigations [13]. In particular, astudy of domestic hot water consumption in 124 dwellings in Eng-land revealed that the mean household hot water consumption is122 L/day, with a 95% confidence interval of ±18 L/day [48]. Thesame study demonstrated month-to-month consumption varia-tions in the range �20 L/day (minimum, July) and +10 L/day (max-imum, December), or between �16% and +8% respectively.Introducing these variations would not influence the results inthe present modelling work, since the thermal energy output ofthe PVT system is found to be always lower than the instantaneoushousehold demand by more than this amount (see Fig. 8(b)). Evenso, a real system may experience significant instantaneous hotwater-related performance variations due to a mismatch betweenthe instantaneous supply and demand.

The study in Ref. [48] also found a hot water delivery tempera-ture with a mean value of 51.9 �C ± 1.3 �C, although normally a boi-ler is expected to provide water at 60 �C [26,39]. In addition, thewater mains temperature was reported as 10 ± 2.6 �C [40], so a va-lue of 10 �C was used in the present work. A representative averagedaily profile is sought that can be repeated each day for simplicity[11,39]. Given the lack of a universal profile, it was deemed reason-able to employ the average profile of hot water consumption foundin the study undertaken in the UK (in Fig. 4(b)) [48]. This is in linewith related studies that use an approximate estimate of the dailyconsumption for a family of four of 120 L at 50 �C [13].

(a)

Fig. 5. (a) Diurnal solar irradiance and (b) average ambien

4.2. Solar irradiance availability

The Photovoltaic Geographical Information System (PVGIS) on-line tool is a map-based inventory of solar energy resources [49].This database was used to obtain solar intensity profiles over thecourse of an average day in each month with a 15-min resolution,giving as outputs the global irradiance, J (W/m2), on a fixed user-defined plane, along with the ambient temperature. Each value re-turned represents an average measurement over a 10-year period(1981–1990). A 36� (to the horizontal) inclination angle andSouth-facing orientation were selected, since they define an opti-mally tilted plane for maximum annual solar irradiation in London.Averages between two consecutive values were used to obtain the30-min resolution input data required by our model. Fig. 5 showsthe solar irradiance and ambient temperature for different months.Significant differences are observed between winter and spring/summer, with the latter having almost three times higher irradi-ance. This is expected to affect strongly the outputs of the PVT sys-tem during the different seasons.

4.3. Environmental assessment parameters

In order to undertake this assessment, the CO2 equivalentemissions associated with grid electricity and natural gasburning are required, as applied to the UK. The values assignedto these parameters in this work were taken from Ref. [58]:0.5246 kg CO2(e)/kWe h for electricity and 0.1836 kg CO2(e)/kWth hfor natural gas.

The average annual end-user demands for hot water heatingand electrical power (which are known, from Fig. 4) are differentfrom the average annual household demands for natural gas (fromthe mains) and electrical power (from the grid). This is because

(b)

t temperature in January, April and August in London.


although all electrical demand is covered by the grid, the heatingdemand is, on average, satisfied not only (although mostly) by nat-ural gas, but also to some extent by the grid (with electrical heatersand heat pumps). In this study the end-user heat and power de-mands have been converted to household gas and electrical de-mands by considering UK-specific data on the current uptake ofboilers, air and ground source heat pumps, and electrical heatersand their respective efficiencies or Coefficients of Performance(COPs). Based on these data, the following expressions have beenobtained:

Q gas ¼0:970:88

� Qaux; Egrid ¼ 1þ 0:031:5� Q aux

Enet

� �� Enet; ð33Þ

where Qaux and Enet represent the auxiliary heat and the electricityrequired to cover the demand, or in other words the short-fall be-tween the demand and the amount produced by the PVT system.These expressions can also be used to calculate the CO2 emissionsincurred to cover these demands.

The following data were used to obtain these factors:

� About 3% of the total boilers in the UK are electrical heaters andheat pumps, with the two systems being employed equally(typically about 20000 new units per year in 2010). Air sourceheat pumps are more commonly used by about a factor of 3times, relative to ground source equivalents [50].� The average boiler efficiency is 88% [51]. This value is domi-

nated by the vast majority of high-efficiency condensing boilers(98.9% of all newly installed boilers in the UK).� The typical COP of an air source heat pump in the UK is 1.75

when producing hot water [52].� The typical COP of a ground source heat pump in the UK is 3.16

[53].� Thus, a value of 2 was taken as the weighted average heat pump

COP, and a value of 1.5 as an average conversion factor fromelectricity to heat considering electrical heaters and both typesof heat pump.� Hence, 97% of households will have a conversion of 0.88 from

fuel (gas/liquid/solid) to heating and 1 for direct use of electric-ity, while the rest of the 3% of the households will have nogas/liquid/solid heating, and a conversion of 1.5 from electricityto heat and 1 for direct use of electricity.

(a)

Fig. 6. Variations over the course of an average day in July of: (a) the temperatures of theof the inlet (Ttin) and outlet (Ttout) temperatures of the heat exchanger in the tank. Resultsflow-rate of VP = 108 L/h, with the value of P suggested by a previous PVT unit study [19[25], respectively. N.B.: TPV (cross ‘�’ signs), Tab (plus ‘+’ signs) and Tbw (circles ‘o’) in Fig.similarly Ttout (diamonds) and Tcout (squares) in Fig. 6(b) either overlap exactly when thethe bypass is deactivated (between 9 am and 6 pm) as would be expected when the flowcase shown here); refer also to Fig. 3.

5. Results and discussion

Numerous variables are set when designing and constructing aPVT unit, and several other parameters are varied to modify its per-formance during operation. In the present work, two parameterswere selected for careful investigation, since these significantly af-fect the performance and outputs of the PVT system: (i) the cover-ing factor of the collector with PV (P) (varied between 0.2 and 1);and (ii) the cooling water flow-rate through the collector (VP)(varied between 0.01 L/h and 200 L/h). In this section results arepresented that were generated when the model described inSection 3 was run with the input parameters, variables and profilesoutlined in Section 4. Firstly, in Section 5.1, a study of the perfor-mance of the PVT unit over the course of a day is undertaken, iden-tifying the different modes of operation, after which theassessment is extended to cover a whole year in Section 5.2. Thiseffort takes the form of parametric analyses, whose aim is to verifythe influence of key parameters (P, VP) on the overall systemperformance. Finally, in Section 5.3, a few selected configurationswith different values of P and VP are compared, also with respectto environmental factors (i.e. CO2 emissions). The aim here is, byselecting the values of these parameters, to identify the mostfavourable configuration in terms of hot water and electricitydemand coverage, whilst maximising CO2 emission savings.

5.1. Diurnal PVT system performance

5.1.1. System operation modesFirstly, the PVT system’s operation is inspected over a typical

day, for which a weekday in July is chosen. The solar irradiance,ambient conditions, hot water and electricity demands over thecourse of this day are taken as model inputs. In addition, an esti-mate of the temperature of the water in the tank (Tt) at the begin-ning of the day (i = 1) is required as an initial condition. The modelis run for several days until the temperature at the beginning (i = 1)and at the end of the day (i = 48) are the same; this is found to oc-cur when Tt = 34.75 �C.

Fig. 6(a) shows the daily temperature variations of the PVT unitlayers, with P = 0.75 and VP = 108 L/h. These settings were takenfrom previous studies (P) [19], and the manufacturer’s operationalspecifications (VP) [25]. It is observed that the sky temperature(Tsky) is significantly lower than the ambient temperature (Ta),

(b)

different layers of the PVT unit and (b) the water storage tank temperature (Tt) andcorrespond to a PVT-system with a coverage factor of P = 0.75 and a collector cooling] and the value of VP recommended by the PVT unit’s manufacturer, ENERGIES-SOL6(a) follow each other very closely, as expected for adjacent contacting layers, andbypass is active (before 9 am or after 6 pm), or follow each other very closely whenleaving the collector enters and exits the tank at a relatively high flow-rate (as is the


leading to radiative heat transfer from the glass to the sky. As aconsequence of these losses, and also the convective losses fromthe glass to the air due to wind, early in the day when the irradi-ance is near-zero (before 6 am, or i = 12; see Fig. 6(a)) the temper-atures of all layers (glass, PV laminate, absorber) and water flow(Tbw) temperature decrease, with the glass temperature (Tg) even-tually falling below the ambient (Ta). From 6 pm onwards there issignificant solar irradiance. The temperatures of all layers gradu-ally increase, as does the temperature of the collector cooling flow.Thus, the flow bypass valve closes (at 8:30 am, or i = 19; seeFig. 6(b)) and the water in the tank is heated. At dusk, when the so-lar irradiance falls to very low values and the ambient temperaturedecreases, the unit cools and the temperatures decrease, asexpected.

The temperatures of the water stored in the tank (Tt) and that atthe inlet (Tcout) and outlet (Ttout) of the heat exchanger in the tank(see Fig. 3) are shown in Fig. 6(b). The system model generates re-sults as expected. Early in the day, the temperature of the waterexiting the collector Tcout is lower than the tank temperature Tt

and the system operates in bypass mode, i.e. the water is sent fromthe collector’s outlet back to its inlet to be re-heated. The tank tem-perature Tt decreases due to losses and demand withdrawal. Then,from 6 am, the solar irradiance causes the temperature of thewater exiting the collector, Tcout, to increase, and from 8:30 am,when this temperature exceeds the tank temperature Tt, the by-pass valve sends the flow to the tank whose temperature startsto rise, while always remaining below that of the water exitingthe heat exchanger. At the end of the day the temperature of theflow leaving the collector Tcout decreases due to the low irradiance;when it drops below the tank temperature Tt, the bypass is acti-vated again so as not to cool the stored hot water.

In light of the results shown in Figs. 6 and 7, four modes can bediscerned in the operation of the PVT system over the course of aday, which are crucial in devising successful control strategies forsuch systems:

1. For i 6 12 (before 6 am) and i P 36 (after 6 pm) in Fig. 7, whenthere is no solar irradiance and the temperature of the PVT unitis lower than the temperature of the water entering the collec-tor, i.e. Tcout � Tcin < 0: In this case it is not beneficial for thepump to be operating, thus sending the water to the collector,as the water is warming the PV module, while electricity is con-sumed in running the pump.

Fig. 7. Temperature rise of the water stream through the collector (Tcout � Tcin),temperature drop of the water stream through the heat exchanger in the hot waterstorage tank (Tcout � Ttout), and heat addition from the collector to the tank ( _qct) overthe course of an average day in July. Corresponding to the same run as the results inthis figure.

2. For 13 6 i 6 18 (6:30 am to 9 am) in Fig. 7, when the water tem-perature at the exit of the collector is higher than that at its inlet(the flow is cooling the PVT unit) yet lower than the tempera-ture in the hot water storage tank (the flow would cool thewater in the tank if it was sent there), i.e. Tcout � Tcin > 0 andTcout � Ttout = 0 (or _qct ¼ 0): In this case, the water pump isrunning and the bypass is active so the collector flow isre-circulated through the PVT unit without heat addition tothe storage tank.

3. For 18 6 i 6 33 (9 am to 4:30 pm) in Fig. 7, when the water tem-perature at the exit of the collector is higher than that at its inlet(the flow is cooling the PVT unit) and also higher than the tem-perature exiting the heat exchanger in the hot water storagetank (the flow is heating the water in the tank), or Tcout � Tcin > 0and Tcout � Ttout > 0 (or _qct > 0): The pump is running with thebypass deactivated, sending the flow from the collector to thetank heat exchanger and heating the water in the storage tank.

4. For 34 6 i 6 35 (5 pm–5:30 pm) in Fig. 7, when the temperaturedifference in the flow across the collector Tcout � Tcin is smallerthan the temperature difference in the tank heat exchangerTcout � Ttout: In this case the bypass is again deactivated andthe pump is running because, although the collector flow inlettemperature increases with time, it is still beneficial to runthe system as heat is still being extracted from the collector,further heating up the water in the storage tank.

The fluctuations in the heat added to the tank _qct that appear inFig. 7, mainly in the interval 9 am–2 pm (i.e. i = 18–28), are due to acombination of the high demand of hot water at this time of theday (Fig. 4(b)) and the low temperature of the water in the tank(Fig. 6(b)). Due to the high demand, hot water is withdrawn fromthe storage tank and replaced by cold water at the lowest temper-ature of 10 �C, thus introducing strong fluctuations in the temper-ature of the water in tank and also in the heat transferred to thetank.

The corresponding electrical and thermal outputs of the PVT sys-tem throughout the day are shown in Fig. 8. Fig. 8(a) indicates thatat some parts of the day when the electrical output is sufficientlyhigh, the household demand is completely covered (Eneti > 0;squares), thus there is a surplus of electricity that can be sold tothe grid. In addition, Fig. 8(b) reveals that the total domestic hotwater demand (diamonds) is not completely covered at any point,however, the hot water provided by the system (squares in the plot)covers 55% of the total demand. We note that at certain times with ahigh solar irradiance the electrical demand also experiences signif-icant peaks, which cannot be covered by the PVT output (Eneti < 0).During periods without solar irradiance electricity is imported,since electricity storage (batteries) is not considered in this work.It is estimated that about 82% of the electrical demand is coveredwith this PVT system over a day in July.

5.1.2. Model validationFigs. 7 and 8 also serve an additional purpose, in validating our

PVT unit model. At the peak irradiance of J = 540 W/m2 (at around12 noon) in the average July day, the thermal output from the sys-tem reaches a peak of approximately 1050 W (Fig. 7) and an electri-cal output 830 W (Fig. 8(a)) from our 15 m2 collector array, or athermal output of 70 W/m2 and an electrical output 53 W/m2.These correspond to efficiency values of 13% and 10%, respectively.Now, the electrical output of the ENERGIES-SOL PVT unit is rated bythe manufacturer at 250 Wp over an area of 1.62 m2 at 1000 W/m2

(Table A1) [16], which gives a 15% electrical efficiency. In addition,the thermal output is 867 Wp for a water flow-rate of 108 L/h [16],which corresponds to a thermal efficiency of 54%, and a water tem-perature rise through the collector of 7 K, or a reduced temperatureof (Tcin � Ta)/J < 0.001. Using a return temperature to the collector of

(a) (b)

Fig. 8. Instantaneous (a) electrical and (b) thermal energy produced by the PVT system over the course of an average day in July. In (a) showing total production and netproduction after accounting for the household demand. In (b) showing total household demand (from Fig. 4(b)) and production by the PVT system. Energies shown here overa 30-min time step. Corresponding to the same run as the results in Figs. 7 and 8.


60 �C at J = 540 W/m2, we would expect (Tcin � Ta)/J = (60 � 25)/540 = 0.065 during July operation. From a typical thermal efficiencyperformance curve for a sheet-and-tube PVT/water collector [54],the thermal efficiency is expected to be within the range �10–15%.

5.1.3. Effect of collector flow-rateThe average solar irradiance over the entire year was calculated,

and the month with the closest average irradiance was selected as anaverage month, which for London was found to be March. For thismonth, the collector flow-rate VP was varied from zero (no waterflow through the collector) to 200 L/h, with higher resolution inthe range 0–40 L/h, while the covering factor was set to 0.75 as sug-gested in Ref. [19], and as employed previously in Figs. 6–8. Fig. 9(a)shows that the hot water demand covered, DCHW, exhibits a signifi-

(c)

(a)

Fig. 9. (a) Electrical, hot water and average demand covered, (b) electrical demand coverweekday in the average month (March) for 75% covered (P = 0.75) PVT systems with dif

cantly greater sensitivity to the collector flow-rate VP, compared tothe electrical demand covered DCE. The hot water demand coveredDCHW has an optimum at around 10–20 L/h. On the other hand, oncloser inspection of the electrical demand covered DCE (Fig. 9(b)) itis observed that the optimum value of collector flow-rate VP thatprovides the maximum electricity can be found between 60 and120 L/h, although it should be noted that the range of variation ismuch smaller than that for the hot water coverage (2% vs. 30%).

It is possible to explain the slight differences in the variations ofthe hot water and electrical demands covered. When there is nowater flow through the collector the demand covered DCHW is zero.As the collector flow-rate VP is gradually increased, the covered de-mand similarly increases, yet at high flow-rates the temperaturereached at the outlet of the collector is low, such that it cannot

(d)

(b)

ed, (c) maximum temperature of the PV and (d) average PV module efficiency over aferent collector flow-rates VP.

Fig. 10. Electricity generated over the course of a day in the average month (March)by fully-covered (P = 1) PVT systems with collector flow-rates VP = 20, 80 and 160 L/h,and comparison with the PV-only system.


be used to heat the water in the storage tank, so the hot water de-mand coverage decreases again. The variation in the electricaldemand covered DCE can also be explained in terms of a combina-tion of factors. At intermediate collector flow-rates VP (around�100 L/h), the maximum PV module temperature TPVmax is low(Fig. 9(c)), which allows a high PV module efficiency gel of15.3–15.4% (Fig. 9(d)) and a correspondingly high electrical de-mand coverage (Fig. 9(b)). However, at even higher collectorflow-rates, the PV efficiency is not further enhanced and displaysa degree of saturation, yet the energy required to drive the collec-tor pump is now higher, and the improvement in the PV efficiencydoes not offset the penalty of running the pump. Therefore, thecovered electrical demand decreases again.

(a)

(c)

Fig. 11. For selected PVT configurations: (a) electrical and hot water demand covered oveand weighted average demand covered over a year for different values of VP and (d) for

5.1.4. Comparison with a PV-only systemFig. 10 shows the energy produced throughout a day in the

average month (March) for a PVT system with three different col-lector flow-rates, VP, and for a PV-only system. The maximum en-ergy produced by the PVT system increases beyond that of the PVonly unit, although the differences among the PVT configurationsstudied are small. This is important, as together with Fig. 9 it sug-gests that the increase in the electricity produced at high collectorflow-rates (VP = 160 L/h) may not compensate the decrease in thehot water demand covered. It is concluded that the collectorflow-rate affects strongly the overall hot water and electrical deliv-ery performance of the PVT system and its effect should be studiedover a complete year.

5.2. Annual parametric analyses

5.2.1. Nominal PVT arrangementWith the role of the collector flow-rate (VP) clarified, a series of

parametric analyses can be undertaken over an entire year of oper-ation. The aim here is to select appropriate values for both P and VP

that maximise the electrical (DCE) and hot water (DCHW) demandscovered, while maximising the CO2 emissions savings. The resultsfrom this effort are shown in Figs. 11 and 12. It is observed, onceagain, that the covered electrical demand is not notably affected(<5% variation) by changes to the collector flow-rate, VP, whereasthe covered hot water demand is significantly affected, decreasingby �35% as VP increases from 20 L/h to 200 L/h. This is due to thelow collector temperatures reached at higher VP, therefore requir-ing a greater use of the auxiliary heater to supply the hot water at60 �C to the house. On the other hand, the electrical output of thePVT system increases linearly with the increase in P (Fig. 11(b)),due to the proportionally larger surface area of the PV module.However, as the PV module area increases, there is less absorber

(b)

(d)

r a year for different values of VP and (b) for different covering factors, P. (c) Averagedifferent covering factors, P.

(b)(a)

(d)(c)

Fig. 12. For the same selected PVT configurations in Fig. 11: (a) separate contributions towards CO2 emission savings due to electricity and hot water production over a yearfor different values of VP and (b) for different covering factors, P. (c) Total CO2 emission savings from both electricity and hot water production over a year for different valuesof VP and (d) for different covering factors, P.

(a) (b)

Fig. 13. (a) Percentage of electrical (E), hot water (HW), average (Avg) and weighted average (W/Avg) demand covered and (b) electrical demand covered throughout the yearfor different values of VP for a PVT system completely covered by PV (P = 1; i.e. the total surface area of the solar collector is covered entirely by the PV laminate).


plate area directly exposed to the solar irradiance (which has ahigher absorptivity than the PV laminate), so the heat transferredto the water flowing through the PVT collector decreases, dimin-ishing the amount of hot water demand covered. Together, the re-sults indicate that high covering factors P are desirable in order tomaximise the electrical output, although the hot water productiondecreases, but to a smaller extent.

One can conclude that the collector flow-rate VP does notstrongly influence the electrical output, but it does affect the hotwater output, while the covering factor P affects the electricaloutput considerably more than it does the thermal equivalent.Thus, and from the average/combined measures of demands cov-ered in Fig. 11(c) and (d), one can suggest high coverage values(P = 0.8–1) and the use of low flow-rates (VP = 20–80 L/h) as being

appropriate in terms of adequately covering both the electrical andthermal demands.

Fig. 12 indicates that the CO2 emission savings due to PVT hotwater production are more significant than the equivalent emis-sion savings due to electricity production. Nevertheless, the totalpercentage of emission reductions is more sensitive to the electri-cal than the thermal emissions, due to the fact that the contribu-tion of electricity generation towards the total emissions ishigher than that associated with hot water production. Further-more, while the electrical emission reductions do not vary appre-ciably for different VP, the emission reductions due to hot waterproduction decrease strongly as VP increases, due to the loweramount of net heat added to the tank, which means that more aux-iliary heat is required. Therefore, low collector flow-rates can


achieve a higher percentage of total emission savings. The emis-sions savings due to PVT electricity production increase as the cov-ering factor P increases, while those due to hot water productiondecrease. Still, since the CO2 emissions due to electricity produc-tion are significantly larger than those for hot water production,the total emission reductions follow the electrical trend, suggest-ing the use of high covering factors.

It can be concluded, both from an electrical and thermal deliv-ery perspective and with regards to emissions, that high coveringfactors P and low collector flow-rates VP are recommended forour PVT system. In order to observe closely the PVT performanceat low flow-rates, a more detailed parametric analysis was per-formed in which the flow-rate was varied down to very low values,over the full range 0–200 L/h, for a covering factor of 1. Fig. 13shows that the collector flow-rate that maximises the hot waterdemand covered is 20 L/h, while the electrical demand covered ismaximised at 160 L/h. However, the range of variation of the latteris much smaller than the former, and as a consequence the collec-tor flow-rate that maximises both the average and weighed aver-age demand covered is 20 L/h (highlighted as bold and italic inTable 1).

5.2.2. Variations in the PVT system configurationThe results from the parametric analysis demonstrated that

high covering factors P and low collector flow-rates VP are prefer-able in maximising the coverage of electricity and hot water de-mands throughout the year, while at the same time maximisingthe CO2 emissions savings. In order to gain insight into the detailedcombined performance of the PTV system throughout the year, fivecombinations of collector covering factors and flow-rates are se-lected, and considered in detail in this section:

� System featuring a high electrical performance unit design,operated for a high electrical output: Covering factor P = 1 andcollector flow-rate VP = 160 L/h.� System featuring a high electrical performance unit design,

operated for a high thermal output: Covering factor P = 1 andcollector flow-rate VP = 20 L/h.� System featuring a high thermal performance unit design, oper-

ated for a high thermal output: Covering factor P = 0.6 and col-lector flow-rate VP = 20 L/h.� System featuring a high thermal performance unit design, oper-

ated for a high electrical output: Covering factor P = 0.6 and col-lector flow-rate VP = 160 L/h.� Intermediate solution system: Covering factor P = 0.8 and col-

lector flow-rate VP = 80 L/h.

The first combination was chosen to represent a system with aPVT unit that exhibits the best electrical performance (high P),operated so as to maximise its electrical output (high VP), andthe second combination is the same system operated to maximiseits thermal output (low VP). Similarly, the third combination repre-sents a system with a PVT unit that exhibits the best thermal per-formance (low P), operated so as to maximise its electrical output(high VP), and the fourth is the same system operated to maximiseits electrical output (low VP). The final combination is an interme-diate solution in terms of design and operation.

Table 1Percentage of electrical, hot water, average and weighted average demand covered for diffevalues highlighted as bold and italic.

VP (L/h) 0 1 2 4 5 6 8 10

DCE 48.75 49.31 49.68 50.11 50.22 50.30 50.42 50.49DCHW 0.00 22.11 26.81 32.30 33.74 34.73 35.90 36.50DCav 24.37 35.71 38.24 41.20 41.98 42.52 43.16 43.49DCwav 30.85 39.32 41.28 43.57 44.17 44.59 45.09 45.35

Fig. 14(a) shows that the electrical production per month fol-lows a profile similar to the solar irradiance, with a more than 3times higher output in the summer compared to the winter. It isalso possible to conclude that the units with covering factor equalto unity produce significant more electrical output than the PVTunits with lower covering factors in summer months. Yet, thereis little difference between the electrical outputs of the systemswith different collector flow-rates. The net power imported or ex-ported by the house in each month (Fig. 14(b)) shows similar re-sults. The net electrical output varies significantly depending onthe month, also given the higher electricity demand in wintermonths when the PVT electrical output is also lower, thus requir-ing more energy to be bought from the grid (negative values ofEPVTnet). It should be noted that for PVT units with P < 1, the net en-ergy is always negative, which means that electricity is always bebought from the grid at the end of each month, while for P = 1, thetotal net energy per month is positive during the summer months,when electricity can be sold to the grid, generating revenue for thehousehold.

In terms of hot water production (Fig. 14(c)), both the coveringfactor and the collector flow-rate influence the output of the sys-tem. The best results throughout the year are found with a cover-ing factor of 0.6 and a 20 L/h collector flow-rate, while the worstresults are given when a covering factor of 1 and a collectorflow-rate of 160 L/h is considered. Hence, it can be concluded thatthe slight improvement in electrical output at higher flow-ratesobserved in relation to Fig. 14(a) and (b) may not outweigh the de-crease in hot water production.

Finally, in terms of CO2 equivalent emissions, the emissionsassociated with electricity production are always greater thanthose associated with hot water (Fig. 15). Depending on the config-uration considered, both vary by up to 10–15 kgCO2 per month,with a greatest deviation in the summer and smallest in the winter.

5.3. Overall assessment and comparison with a PV-only system

In this section the PVT configurations selected above are com-pared with each other as well as with a PV-only system in termsof demand covered and CO2 emissions saved over the lifetime ofthe PVT system, considering two different lifetimes (n), 20 years[11] and 25 years [55]. The results of this comparison are summa-rised in Fig. 16 and Table 2. Table 2 shows that a complete coverageof the collector with PV is preferred in terms of electrical, averageand weighted average demand covered, as well as in terms of thetotal CO2 emissions saved, whereas the lowest coverage factor(P = 0.6) has the highest thermal output and thermal demand cov-ered. Still, the thermal output of the fully covered ‘high electricalperformance’ PVT system can be significantly increased (by >40%)by using a lower collector flow-rate VP if this is desired from thehousehold, which can be achieved in a practical system with theuse of a variable-flow pump, whereas the electrical output of the‘high thermal performance’ system only marginally improves (by�5%) by increasing the cooling flow from VP = 20 L/h to 160 L/h.Fig. 16 confirms that a high P (=1) and a high VP in the PVT collectorare preferred for maximising the covered electrical demand, whichallows a slightly higher electrical output than the PV-only system.However, the hot water demand covered decreases strongly at high

rent collector flow-rates for a PVT system completely covered by PV (P = 1). Maximum

20 30 40 50 80 120 150 160 200

50.75 50.97 51.18 51.38 51.77 52.01 52.04 52.07 52.0336.60 35.52 34.30 33.10 30.16 27.47 26.23 25.68 24.4343.67 43.25 42.74 42.24 40.96 39.74 39.14 38.88 38.2345.55 45.30 44.98 44.67 43.83 43.00 42.57 42.38 41.90

(a) (b)

(c)

Fig. 14. (a) Total energy produced by the PVT unit, (b) net energy imported/exported by the house per month and (c) thermal energy produced by the PVT unit per month, forselected PVT configurations. All figures have the same legend as in (b).

(b)(a)

Fig. 15. CO2(e) emissions due to (a) power and (b) hot water consumption per month, for selected PVT configurations. Legend relates to both plots. Both figures have the samelegend as in (a).


VP, while the improvement in the electrical output is only minimalcompared to lower VP; therefore the best compromise seems to bea collector flow-rate of VP = 20 L/h, with which 50.7% and 35.6% ofthe electrical and hot water demands, respectively, are coveredover the course of a full year (see Table 2). Finally, this is alsothe best configuration in terms of CO2 emissions reduction, as16 tonnes of CO2 can be saved over a lifetime period of 20 years,compared to the 11.8 tonnes of CO2 saved with a PV-only systemof the same capacity. Importantly, all selected PVT configurationsoutperformed the PV-only system in terms of emissions, by up to36% in the best PVT case.

5.4. Brief economic considerations

At this stage, it is important to perform a brief economic analy-sis relating to the deployment of the proposed PVT systems in the

UK domestic sector. A number of representative configurationswhere selected for this purpose, with collector flow-rates: (i)VP = 20 L/h, (ii) VP = 40 L/h, (iii) VP = 80 L/h, and (iv) VP = 160 L/h,all with a fully covered PVT collector, P = 1, since it is found inthe present work that this is the most economically beneficial con-figuration for the chosen application of this technology. The initial/capital expense was estimated by summing the individual costs ofall system components, which in all cases was found to amount to£8500 for a 2.25 kWp (electrical) PVT system with a 150 L hotwater storage tank, including (in addition to the PVT collectorand tank) the inverter, pump, structural supports, piping, wiring,installation, etc. After accounting for the running costs of conven-tional equivalents (buying electricity from the grid and natural gasfor a modern high-efficiency boiler) and any income from thecurrently available incentives, i.e. (i) the Feed-in Tariffs (FITs); (ii)the Renewable Heat Premium Payment (RHPP); and (iii) the

Fig. 16. Percentage of electrical (E), hot water (HW), average (Avg) and weightedaverage (W/Avg) demand covered for selected PVT configurations and comparisonwith the PV-only system.


Renewable Heat Incentive (RHI), which is expected to becomeavailable to the UK domestic sector shortly, it was found that thepayback periods of the PVT systems ranged from 10 to 12 years,also depending on the discount (2.5–10%) and inflation rates(3.2–6%) used in the calculation. The following data was used inarriving at these estimates: (i) an electricity price of 17.4 p/kW h;(ii) a natural gas price of 5.8 p/kW h; (iii) a FIT payment for PV of43.3 p/kW h; (iv) a one-off £300 RHPP voucher for the installationof a solar-thermal product; and (v) a RHI payment of 8.5 p/kW h,all for the specific case of the UK. The PVT system payback periodswere compared with an equivalent 2.25 kWp PV-only system,which was evaluated by using an identical approach. The paybackperiod of the PV-only system was found to be approximately7 years at current levels and means of support, although it is notedthat this can only cover an electrical demand, as opposed to a PVTsystem that can cover this as well as a demand for hot water. Gen-erally, the selected PVT systems were found to be very closelymatched in our assessment, with the VP = 20 L/h configurationshowing a marginally lower payback.

6. Further discussion and conclusions

The current market for both solar thermal and PV systems isexperiencing significant growth, owing to the recognition of thesetechnologies as key and necessary measures for sustainable energyprovision and carbon mitigation [4]. This growth can be attributedto incentives derived from government policy support and theincreasing environmental awareness of the end-user. Consequently,the integration of these solar-based technologies to form a hybridPVT system has considerable potential in this scenario, not only incombining the advantages of the separate units into a single systemable to provide both electricity and hot water, but also in the syner-

Table 2Summary of the performance and the environmental assessment for selected PVT configuraand italic.

P = 1; VP = 160 L/h P = 1; VP = 20 L/h P

EPVT (kW h) 2392 2290 1QPVT (kW h) 670 955 1DCE (%) 51.8% 50.7% 3DCHW (%) 23.3% 35.6% 4DCav (%) 37.5% 43.2% 3DCwav (%) 41.3% 45.2% 3EmsT (kg CO2(e)/yr) 740 800 6Tonnes CO2 saved n 20 14.8 16.0 1

n 25 18.5 20.0 1

gistic manner in which the heat removal for hot water provisioncools and increases the efficiency of the PV cell. Therefore, a similargrowth in the demand for PVT systems is expected, with the domes-tic sector almost certainly forming the largest market share (cur-rently �90% of the market according to Ref. [14]). Policy-makershave a crucial role to play in boosting the uptake of these technolo-gies by means of establishing them as cost-competitive and com-mercially attractive alternatives to conventional solutions [4].

The present effort focuses on the performance of PVT/water sys-tems in the domestic sector, in order to examine their suitabilityfor the distributed generation of electricity and simultaneous pro-vision of hot water in this application. Different solar-thermal col-lector configurations were reviewed in a number of excellentprevious studies [8–11,15,22]. These studies concluded that thesheet-and-tube configuration is a highly appropriate option interms of both electrical and thermal efficiency, and also that thisis the easiest and most affordable configuration to manufactureas it relies on well-known, readily available technology. In addi-tion, mono-crystalline (c-Si) PV cells are often selected by reasonof their higher electrical output (efficiency), as well as their greaterstability compared to both amorphous and poly-crystalline modulecounterparts. All commercially available PVT units that have beenidentified in the present study employ c-Si.

In hybrid PVT systems the total energy output (electricity plusheat) depends on several factors and typically there is a conflict be-tween the electrical and thermal performance [8,10,12,15]. Thepresent study has investigated two key system parameters thatstrongly influence the output of the PVT system: the coolingflow-rate through the collector unit; and the covering factor ofthe solar collector with PV cells [21]. The aim was to maximisethe supply of both electricity and hot water in the particular sce-nario of an average 3-bedroom terraced house in London, UK[42], while also maximising the total CO2 emission savings. Tothe best knowledge of the authors the novelty of the present effortarises from it being the first UK-based effort of its kind, followingon from excellent investigations of other configurations and inother geographical regions, such as in Refs. [56–58]. Of particularinterest in the present work has been the consideration of thewhole system (PVT unit, hot water storage tank, auxiliary heaterand the household), with varying daily temporal profiles of solarirradiance and household demands, over the course of an entireyear, including a parametric analysis and attempt to identify bet-ter-performing configurations and operating conditions/strategies.

The main findings regarding the covering factor are: (i) that itsignificantly influences the electrical output, such that the percent-age of electrical demand covered increases notably with the cover-ing factor, while it only has a minimal effect on the thermal output;and (ii) that the total CO2 emissions savings are more affected bythe electrical output, due to the fact that emissions associated withelectricity generation are much higher than those associatedwith hot water production. On the other hand, the main findings

tions and comparison with the PV-only system. Maximum values highlighted as bold

= 0.6; VP = 20 L/h P = 0.6; VP = 160 L/h P = 0.8; VP = 80 L/h PV-only

355 1428 1876 2193127 740 835 00.0% 30.6% 41.2% 48.7%2.0% 28.4% 32.0% 0.0%6.0% 29.5% 36.6% 24.4%4.4% 29.8% 37.8% 30.9%88 607 709 5893.8 12.1 14.2 11.87.2 15.2 17.7 14.7

Table A1Technical specification of the modelled PVT unit (single module) [17].

Nominal power 250 Wp

Total surface area 1.624 m2

Total aperture area 1.566 m2

Voltage at Maximum Power Point (MPP) 30 VCurrent at MPP 8.34 AOpen circuit voltage 36.9 VShort circuit current 8.34 APressure drop at the recommended flow-rate 150 mbarMaximum operating pressure 3.5 barRecommended flow-rate 108 L/hReference PV module efficiency 15.4 %Temperature coefficient of cell power (b0) 0.53 %/KNormal Operating Cell Temperature (NOCT) 45 ± 0.2 �CType of solar cell Mono-crystalline (c-Si)


with regards to the collector cooling flow-rate are: (i) that the per-centage of hot water demand covered is considerably more sensi-tive than the electrical demand covered to the increase in thecollector flow-rate, with the former decreasing at higher flow-rates; and (ii) that the emissions savings that arise from the hotwater output are affected by the increase in the collector flow-rate(due to the increase in auxiliary heat at higher the collector flow-rates), but the total CO2 emissions savings are not significantly af-fected by this parameter. Consequently, high covering factors andlow collector flow-rates are suggested. With these findings inmind, it can be concluded that the investigated parameters influ-ence strongly the electricity and hot water production, with theformer being more sensitive to covering factor variations, and thelatter to collector cooling flow-rate variations. Similar results werefound when five selected PVT configurations were investigatedthroughout the different months of the year. Specifically, a cover-ing factor in the range P = 0.8–1 and the use of low collectorflow-rates in the range VP = 20–80 L/h are recommended in orderto maximise the overall coverage of the demanded heating andpower throughout the year while increasing the total CO2 emis-sions savings.

Up to 52% of the total annual electrical demand (for a configu-ration with P = 1, VP = 160 L/h) and up to 48% of the annual hotwater demand (for a configuration with P = 0.2, VP = 20 L/h) couldbe covered by such a PVT system. The most suitable configuration(P = 1 and VP = 20 L/h) was selected for direct comparison with aPV-only system consisting of c-Si modules with the same peakcapacity as the PVT system (2.25 kWp). The results indicate that alarger amount of the annual electricity demand is covered by thePVT unit (51% vs. 49%), while also covering 36% of the total demandfor hot water. Consequently, the CO2 emissions are substantiallylower (16 tonnes of CO2 saved vs. 11.8 tonnes, over a lifetime per-iod of 20 years). All investigated PVT configurations outperformedthe PV-only system in terms of emissions. A surplus of electricitycan occur with some PVT configurations during the summermonths, and this is exported to the grid. The improved capacityof the PVT technology to cover the total (both electrical and ther-mal) domestic energy needs and much greater potential for emis-sions abatement compared to PV-only systems does come a highercost, with payback periods estimated in the range 10–12 yearscompared to 7 years for PV. This of course is a strong function ofcurrent UK incentives. We suggest that there is a need to considerthese more carefully in the light of present results.

In summary, we conclude that the configuration of the PVT sys-tem significantly affects its thermal and electrical output, and be-cause is not possible to maximise both outputs at the same time,the values chosen for the main system parameters depend on thespecific end-user needs. On the basis that the CO2 emissions ofelectricity are significantly higher than that of natural gas, the elec-tricity production is usually the priority. In the specific case of theUK where the temperatures reached in the PVT unit are not toohigh, it is recommended to completely cover the solar collectorwith PV, since the loss in electrical efficiency is more than offsetby the higher electrical output allowed by larger PV surface area.In addition, a low collector flow-rate is recommended. However,one should consider that in other geographical locations with high-er solar irradiance it may be necessary to reconsider the preferredcovering factor as well as the collector flow-rate.

This paper has focused mainly on the technical performance of ahybrid PVT/w system in a domestic setting in the UK with only abrief consideration of costs, which are expected to affect uptakedecision-making, for example owing to the higher up-front invest-ment costs required by PVT systems compared to standard PV-onlysystems. Therefore, a more in-depth complementary economicstudy may also be required for a complete assessment of thesuitability of these systems and for a better understanding of the

current effects of government incentives. When strong incentives,such as FITs, are applicable (as is the case currently in the UK) thereis an added benefit in producing surplus electricity because ahousehold featuring a PVT or PV system can then export this elec-tricity and generate an income, thus making the system moreattractive.

Acknowledgement

This work was supported by the Research Councils UK (RCUK)[grant number EP/E500641/1].

Appendix A. PVT system parameters

The PVT system evaluated in this paper is based on the com-mercially available hybrid unit ENERGIES-SOL [16]. Many technicalspecifications required for the modelling of this system, though notall, are available in the specifications sheet provided by the manu-facturer. The rest of the required parameters have been estimatedfrom the literature [3,22,24,30,31]. The total roof area available inan average terraced house in London is about 15 m2 [44]. Since thetotal area of a single PVT module is 1.62 m2, the system consideredconsists of 9 modules arranged and operating in parallel. The mod-ule consists of mono-crystalline (c-Si) cells, which provide a highmodule efficiency of 15.4% at Standard Test Conditions (STC:1000 W/m2 and 25 �C). The temperature coefficient for the PVmodule, b0 (used in relation to Eq. (6)), is 0.0053 K�1. The completePVT system has a nominal electrical power of 2.25 kWp, with eachmodule contributing an equal 250 Wp of nominal electrical power.Important technical specifications are detailed in Table A1.

The manufacturer does not provide details of the design andmaterials used, such the optical and thermal properties of the dif-ferent layers and their geometrical specifications. These parame-ters are important in determining the performance of the PVTunit and are required in the energy balances developed in Section 3to evaluate the system. The values used in this paper of the opticaland thermal properties of the different materials from which thePVT unit is constructed where taken from the literature[3,22,24,26,30,59,60] (see Table A2). Similarly, to calculate the con-vective heat transfer to the cooling water stream flowing throughthe collector, it is necessary to know the geometry of the risertubes. As this information is not provided, it has also been esti-mated from values found in literature for similar systems[3,22,24] (also stated in Table A2).

Once the geometry of the riser tubes is selected, it is possible toestimate the convective heat transfer coefficient of the water flow-ing through the collector. However, it should be kept in mind thatthis coefficient also depends on the water flow-rate through thecollector. Initially, the recommended water flow-rate by the

Table B1Summary of the main parameters of the water storage tank and the immersed heatexchanger.

Mt Storage tank capacity (kg) 150 [31]Ht Height of the tank (m) 1.5 [31]Dt Diameter of the tank (m) 0.36 [31]St Surface area of tank (m2) 1.9 [31]Ut Overall heat loss coefficient (W/m2) 3 [31,62](UA)t Overall heat transfer coefficient (W/K) 573NTU Number of transfer units 0.51e Heat exchanger effectiveness 0.4

Table A3Water pump and collector cooling circuit specifications [61].

HT Total head loss through the system 2.91 mQ Nominal flow-rate 0.972 m3/h

Operating pressure 3.5 barPP Nominal electrical consumption 9.07 WQmax Maximum flow-rate 4 m3/hHTmax Maximum head 6 m

Maximum operating pressure 10 bargP Pump efficiency 0.85 –

Table A2Optical and thermal properties, thicknesses, geometrical characteristics, and other parameters related to the different layers that compose the modelled PVT unit.

Layer Parameter/variable Value Refs.

Glazing A Aperture area (m2) 1.566 [16]d Thickness (m) 0.0032 [16,22,24]e Emissivity 0.88 [3,26,28]s Transmittance 0.9 [31]qd Diffuse reflectance 0.16 [33]

Air layer d Thickness (m) 0.005k Thermal conductivity (W/m K) 0.025 [22]

PV plate APV Area of the cellP Covering factor 0.75q Packing factor 0.9 [60]a Absorptivity 0.9 [59]e Emissivity 0.9 [22]d Thickness (m) 0.00035 [24]

EVA d Thickness (m) 0.0005 [22,30]k Thermal conductivity (W/m K) 0.35 [22,30]

Adhesive d Thickness (m) 5�10�5 [22,30]k Thermal conductivity (W/m K) 0.85 [22,30]

PE-Al-Tedlar layer d Thickness (m) 0.0001 [22,30]k Thermal conductivity (W/m K) 0.2 [22,30]

Absorber plate a Absorptivity 0.95 [3]e Emissivity 0.05 [3]d Thickness (m) 0.0002 [3,24]k Thermal conductivity (W/m K) 310 [3]

Riser tubes D Diameter of tube (m) 0.01 [22,24]N No. of tubes 11 [3,22]W Spacing of tubes 0.095 [22]

Insulation d Thickness (m) 0.02k Thermal conductivity (W/mK) 0.035 [3,22,33]


manufacturer (ENERGIES-SOL) was considered and a value of259 W/m2 K was found. However, it should be noted that the man-ufacturer-recommended cooling flow-rate varies significantlyacross different commercial PVT units, and also in the literature.In Ref. [40] the collector flow-rate is 40 L/m2 h, whereas Refs.[26,27] suggest 50 L/m2 h. While these values are similar, they dif-fer significantly from the manufacturer-recommended flow-ratefor our selected PVT unit, which is 108 L/h or around 70 L/m2 h[16]. Other studies consider a much wider range of flow-rates[30], stating that the optimum flow-rate lies between 18 L/h and270 L/h. Therefore there is a lack of agreement concerning the rec-ommended or optimum flow-rate, which will depend strongly onthe specific needs of the particular application. As a consequence,this work considers explicitly the effect of the water flow-ratethrough the collector on the overall performance of the system.

Another necessary parameter that is required is the pump(hydraulic) power required to drive the collector closed-loopwater-cooling circuit. The recommended flow-rate provided bythe PVT manufacturer has been used as an initial value of thepumping power, in order to select a suitable pump among thoseavailable in the market, for which a full pump curve was obtained.The pump specifications are summarised in Table A3.

Therefore, the values of the convective heat transfer coefficientof the cooling flow through the collector and of the pumping powerare explicitly calculated in the model, once the collector flow-ratehas been imposed.

Appendix B. Water storage tank parameters

The daily hot water demand for the house studied is about122 L, so a water storage tank with a capacity of 150 L was se-lected. To estimate the heat transferred from the collector to thewater contained in the tank, the overall heat transfer coefficientof the heat exchanger immersed in the tank should be calculated,

for which the dimensions of this heat exchanger are required.Table B1 summarises the parameters used. An overall heat transfercoefficient (UA)t of 573 W/K is found based on these values. For dif-ferent collector flow-rates the model automatically calculates thenew number of transfer units NTU and heat exchanger effective-ness e.

There are also some heat losses through the storage tank walls,which are accounted for in the present work. An overall heat losscoefficient for the storage tank, Ut, was taken from the literature.The values found in literature are very similar, so a value ofUt = 3 W/m2 is used [31,62].

Table B2Initial conditions and other important constant system parameters.

Twin Mains water temperature 10 �CTt (i = 0) Initial temperature of water storage tank 34.75 �CTcin (i = 0) Initial temperature of water flowing through the collector 34.75 �CTas Average ambient temperature in the house (tank surroundings) 20 �Chw Convective heat transfer coefficient of water flow through the collector 259 W/m2 K


Finally, initial conditions are required at the beginning of theday (i = 1) for the solution of the equations. These include the ini-tial temperature of the water stored in the tank and the initial tem-perature of the water entering the collector. In addition, twofurther parameters must be defined, which are the mains watertemperature entering the storage tank, and the average ambienttemperature in the house (Tas) that is required to estimate thelosses through the storage tank walls (which is located inside thehouse) (see Table B2).

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